Geodesy can be an intimidating subject. It is based on very complex mathematics that few of us will ever master. As a science, geodesy involves the measurement and study of the size and shape of the earth. This seemingly esoteric discipline, however, is the basis for the geodetic control networks upon which surveying and mapping rely.
In order to place individual surveying projects into a larger spatial context, a geodetic control network is necessary. Such a network consists of a number of points spread across the landscape where sturdy monuments have been placed in the ground, along with a high-accuracy positional value for each point. Traditionally, there have been completely separate networks for horizontal and vertical control, but some networks combine the two on common monuments.
By referencing field measurements to such a network, the resulting data and information from multiple local survey projects can be accurately connected. The accuracy of each project is no higher than the control network to which it is referenced. A control network itself is established by highly precise surveying methods followed by a statistical adjustment to reconcile all of the measurements. For each monument the result is a published positional value along with a stated accuracy level. While control surveying work may be carried out incrementally over time, with the new measurements adjusted to the existing network, adjustments across large expanses are usually performed only periodically, with the resulting values for the control points published all at once. A control point established some years ago may have multiple positional values, each successive value published as part of a fresh adjustment.
The spatial density of geodetic control points is a reflection of costs and benefits. Wide spacing costs less to establish and maintain, but a survey crew has to work over greater distances to connect their project to the control network. As high-end GPS equipment has become more powerful and less costly, a closer spacing of control points delivering more end-user benefits has become more affordable.
Actually, geodetic control is typically separated into two components: horizontal (latitude/longitude) and vertical (elevation). This is because latitude/longitude and elevation are based on completely different concepts and measurement methods. Even today when GPS can provide extremely high-accuracy horizontal results, a more traditional method is required to establish vertical control. While a network point may have geodetic-quality values for both horizontal and vertical, the methods of determining these values will still be different.
National Spatial Reference System
The National Spatial Reference System (NSRS) , formerly known as the National Geodetic Reference System (NGRS), was established in 1994 to better meet the various local needs of surveyors, engineers, and scientists. The National Geodetic Survey (NGS) has primary responsibility for the NSRS. Additionally, the NGS assists with, and coordinates geodetic control surveying activities with other federal agencies and with the all states to establish, upgrade, and maintain NSRS control stations. The NSRS is separated into horizontal and vertical sections. Each control point is classified based on accuracy.
Geodetic control surveys are usually performed to establish a basic control network (framework) from which supplemental surveying and mapping work are performed. The required accuracy for a control survey depends primarily on its purpose. Factors that affect accuracy are type and condition of equipment used, field procedures adopted, and the experience and capabilities of personnel employed. In 1984, and again in 1998, the Federal Geodetic Control Subcommittee (FGCS) published different sets of detailed standards of accuracy and specifications for geodetic surveys.
Horizontal Accuracy Classifications
The 1998 FGCS standard is independent of the method of survey, and is based on a 95% confidence level. To meet these standards, control points in the survey must be consistent with all other points in the geodetic control network, and not just those within a particular survey.
|Accuracy Classifications||95% Confidence |
Less than or equal to
The 1984 FGCS standard(PDF) established three distinct orders of accuracy for traditional control surveys (first-order, second-order, and third-order) with second-order and third-order each having two separate accuracy categories, class I and class II. Three new orders of accuracy were added in 1985 for GPS surveys (AA, A, and B).
|Second-Order, Class I||1:50,000|
|Second-Order, Class II||1:20,000|
|Third-Order, Class I||1:10,000|
|Third-Order, Class II||1:5,000|
High Accuracy Reference Network
In 1988, the Wisconsin Department of Transportation (WIDOT) and NGS jointly established the High Accuracy Reference Network (HARN) for Wisconsin. It was based upon GPS positioning and produced highly accurate results that were the basis for the NAD 83 (91) datum adjustment. It also provided the surveying community with a network of highly reliable positional coordinates to control their surveys.
In 1997, the HARN was resurveyed and readjusted, with approximately 90 stations added. This improved horizontal network not only provides a wide variety of benefits to the Wisconsin land information user community, but it also serves as the foundation for improving the vertical geodetic network as well.
To better support local land information systems, about 44 counties in Wisconsin have added additional control markers based on the HARN. These user-densified networks (UDNs) support accurate mapping and surveying at the local level.
Other counties, such as seven in the southeastern corner of the state have placed horizontal and vertical control values on re-surveyed Public Land Survey System monuments. This data is maintained by the Southeast Wisconsin Regional Planning Commission (SEWRPC).Return to Top
Vertical control networks are a series of points on which precise heights, or elevations, have been established. Vertical control stations are typically called bench marks. As part of a vertical information network, the bench mark's elevation is known relative to a datum, usually mean sea level.
When working with geodetic control points, high accuracy is essential, although the points can be widely scattered. When attempting to model features or processes across the landscape (e.g., flood response to a rainfall event in a watershed), a much denser pattern of reasonably accurate points that together faithfully represent the overall terrain is more important. There is no single technology that can economically yield data that supports both geodetic and landscape analysis needs, and there may never be such a single solution. Nevertheless, current technologies can better meet each of these of needs than just 10 years ago and even more promising technologies are emerging.
Vertical Accuracy Classifications
The vertical accuracy standard as referenced in the 1998 FGCS standard specifies a linear value (plus or minus) within which the true or theoretical location of the point falls 95 percent of the time.
|Accuracy Classifications||95% Confidence|
Less than or equal to
Existing products, including the NGS control data sheets, refer to the 1984 standards, which established three distinct orders of accuracy to govern traditional control surveys (first-order, second-order, and third-order). The first-order and second-order each have class I and class II accuracy categories. These level lines mostly followed railroad corridors. Values were stated relative to NGVD29 , then readjusted relative to NAVD88.
|First-Order, Class I||0.5|
|First-Order, Class II||0.7|
|Second-Order, Class I||1.0|
|Second-Order, Class II||1.3|
Our office has available Standards and Specifications for Geodetic Control Networks, and Geometric Geodetic Accuracy Standards and Specifications for Using GPS Relative Positioning Technology. The NGS has these two publications as well as selected others.
Other Wisconsin standards that relate to geodetic control include Wisconsin Land Information Association Standard 1994-4: Geodetic Control Clearinghouse.USGS
USGS vertical control was published in NGVD29 values, and was not used in the computations that resulted in NAVD88. Approximate conversions between the two datums can be accomplished with VERTICON software.
National Height Modernization
Over the past 200 years, approximately 750,000 precisely located, in-ground or monumented reference points were installed to measure heights. The classical line-of-sight measurements do not provide the real-time accuracy needed for today's positioning technologies and applications.
Through the use of GPS, pinpoint positioning accuracies can be provided 24 hours a day. The combination of an improved national height system, referencing NAVD 88, along with GPS, offers the nation the ability to obtain precise vertical measurements in real time.
In Wisconsin, because much of the existing vertical control has been lost to destruction or disturbance, and because the network was not dense enough to support use of GPS to derive elevations, the Wisconsin Height Modernization Program (WI-HMP) was begun.
Control and the PLSS
Traditionally, neither horizontal nor vertical control networks have been built using Public Land Survey System (PLSS) corner monuments as control points. This is because, prior to GPS, horizontal control was most easily established by measuring between hilltops. Similarly, vertical control was (and still is) most easily established by leveling along railroad or highway corridors. Even once GPS became a practical technology, PLSS monument locations were still problematic as control stations since some are in the middle of roads and others are not easily accessible. In fact, HARN station locations were chosen specifically to be on public land and away from disrupting activities. For instance, a number of HARN stations are located at airports.
Nevertheless, it can be a great advantage in property surveys to have published control values on PLSS monuments, because the surveyor saves the time otherwise required to survey ("run") control in from nearby (sometimes several miles or more away) geodetic stations. This approach is exemplified in the seven counties served by the Southeastern Wisconsin Regional Planning Commission where the standard practice is to install robust monuments at all PLSS corners and then establish moderately accurate values for both horizontal position as well as elevation.Return to Top